This invention relates to grinding wheels containing the high cost premium abrasives: diamond, or cubic boron nitride. It is conventional to construct such wheels in the form an annular section of bonding matrix containing the abrasive, mounted on a central hub or core portion which does not include any of the premium abrasive. It is well known that it is desirable that the core have relatively high thermal conductivity to conduct heat away from the grinding section, good adhesion to the abrasive section, and adequate strength to withstand the grinding forces and the centrifugal forces caused by rotation of the wheel. In addition, and also well known, the core portion must have thermal expansion properties consistent with the thermal expansion properties of the abrasive section, and it must be machineable, grindable, and "dressable" to satisfy the practical requirements of wheel manufacture and use, it being desirable to machine or drill the core for mounting, the grind it to (for example) maintain parallelism between its sides, and to dress it during grinding use by an abrasive dressing stick. It should also be capable of forming a strong bond to the grinding section.
A common type of material for use in making cores is a phenolic resin with a large amount of aluminum powder filler, as taught by U.S. Pat. No. 2,150,886 to Van Der Pyl. In U.S. Pat. No. 3,389,117, the addition of up to 30 weight % of iron powder, or other metal as an alloying element, based on the total weight of the aluminum alloy, is suggested. The metal addition is alleged to improve the dressability of the core, without decreasing its strength or thermal properties, so long as at least 20% by weight of the core is resin. U.S. Pat. No. 4,042,346 teaches substituting another thermosetting resin for a portion of the phenolic resin in an aluminum filled resinous core. It also teaches the use of graphite in the composition as an aid in dressing. British Pat. No. 1,364,178 teaches the simultaneous hot pressing of a diamond rim section, bonded with the high temperature polyimide resin, processable at 350.degree. to 550.degree. C., and an aluminum powder core section. The core section is sintered metal, not metal filled resin.
U.S. Pat. No. 4,042,347, to Richard H. Sioui, teaches making an abrasive section bond of resin and metal, where the processing temperature is high enough such that the metal powder in the mix sinters to form a metal skeleton, and the resin cures to form a resin skeleton in the matrix.
The present invention relates to a composite aluminum-resin core, containing a relatively large amount of a magnetic metal, to provide a core having desirable physical properties and having the ability to be held on a magnetic chuck for finishing operations after completion of the molding of the wheel.
The resin-metal composite of the present invention is processed at such a temperature that the aluminum powder sinters, resulting in a core having additional strength above that provided by the resin bond. Alternatively, powdered zinc or tin may be substituted for the resin, to give an all metal bond.